Leica M-Monochrom has a
18.0MP Full frame (36 x 24 mm ) sized Monochrome CCD sensor . On the other hand, Leica SL has a
24.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro II processor.
Leica SL's sensor provides 6MP more than Leica M-Monochrom's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
One other difference between these two cameras that is worth mentioning is that Leica SL's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the M-Monochrom and SL sensor size comparison.
Sensor Size and Resolution Comparison image of Leica M-Monochrom and Leica SL Cameras
Leica M-Monochrom and Leica SL have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Leica M-Monochrom has 32% larger pixel area compared to Leica SL, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Leica M-Monochrom
or the
Leica SL ,
including hyperfocal distance.